The wealthiest county in Missouri,[3] St. Charles County is one of the fastest growing counties in the nation. The county is also recognized as very conservative, ranking in the top 100 nationally, and many residents support a gun culture.[4]

The County of St. Charles was originally called the District of St. Charles and had no definite limits until 1816 to 1818 when neighboring counties were formed.[5] The borders of St. Charles are the same today as they were in 1818.

I-64 – Major freeway in the western portion of the county. Originally U.S. Route 40, the highway was upgraded to Interstate standards in the late 2000s. The highway was resigned as Interstate 64 from the Daniel Boone Bridge to Interstate 70 in Wentzville in 2009.

I-70 – The major east-west thoroughfare in the county. It is mostly a six-lane freeway throughout most of the county but there are sections in St. Charles and St. Peters where the Interstate widens to 11 lanes of traffic.

As of 2010, there were 360,485 people, 132,906 households, and 77,060 families residing in the county. The population density was 643 people per square mile (1665/km²). There were 142,766 housing units at an average density of 73 persons/km² (188 persons/sq mi). The racial makeup of the county was 91.3% White, 4.4% African American, 0.3% Native American, 2.3% Asian, 0.1% Pacific Islander, and 1.6% from two or more races. Hispanic or Latino of any race were 2.9% of the population.

There were 101,663 households out of which 40.50% had children under the age of 18 living with them, 63.20% were married couples living together, 9.20% have a woman whose husband does not live with her, and 24.20% were non-families. 19.40% of all households were made up of individuals and 5.90% had someone living alone who was 65 years of age or older. The average household size was 2.76 and the average family size was 3.18.

In the county the population was spread out with 29.00% under the age of 18, 8.20% from 18 to 24, 32.60% from 25 to 44, 21.60% from 45 to 64, and 8.80% who were 65 years of age or older. The median age was 34 years. For every 100 females there were 97.10 males. For every 100 females age 18 and over, there were 94.10 males.

The median income for a household in the county was $71,458, and the median income for a family was $64,415. Males had a median income of $44,528 versus $29,405 for females. The per capita income for the county was $23,592. 4.00% of the population and 2.80% of families were below the poverty line. Out of the total people living in poverty, 4.90% are under the age of 18 and 5.10% are 65 or older.

St. Charles County, with an estimated population of 373,495, has been one of the fastest-growing counties in the country for decades, with 55% growth in the 1970s, 48% in the 1980s, 33% in the 1990s, and another 27% in the 2000s. The county features a cross-section of industry, as well as extensive retail and some agriculture. With the Missouri River on the south and east and the Mississippi River on the north, the county is bisected east to west by Interstate 70. St. Charles County has two small airports, St. Charles County Smartt Airport and St. Charles Airport, and two ferries that cross the Mississippi River.

St. Charles County is governed by a county executive and a county council. The county council consists of seven members, each elected from various districts in the county. The county executive is elected by the entire county. The current executive is Steve Ehlmann. He was preceded by Joe Oertwerth, who was preceded by Gene Schwendemann, the first county executive of St. Charles County under the new form of government. The executive under the old form of county government was termed a "judge." The county produced 243,382 registered voters as of 2010.[11]

District 13 – Chuck Gatschenberger (R-Lake St. Louis). Consists of some of Dardenne Prairie and the communities of Flint Hill, Foristell, Josephville, Lake St. Louis, O’Fallon, St. Paul, and Wentzville.

Missouri House of Representatives – District 13 – St. Charles County (2010)

^Korando, Russell (November 23, 2012). "St. Charles County judge nominated for prosecutor". St. Charles County Suburban Journal. Retrieved December 3, 2013. Ehlmann was required to appoint someone from the same political party as Banas, a Republican, to serve until the office comes up for election in November 2014.